Various systems have been proposed for introducing liquid sample into the ion source of a mass spectrometer. One of these systems is disclosed in Japanese Utility Model Unexamined, Laid-Open No. 116065/1986 and schematically illustrated in FIG. 1.
Referring to FIG. 1, the inside of an ion source housing 1 is evacuated. An ionization chamber 2, baffles with slits 3, and a beam generator 4 are mounted inside the housing 1. An inlet tube 5 has one end connected to a liquid chromatograph 6. The opposite portion of the tube 5 extends through an end flange 7 into the ionization chamber 2. A support ring 8 is interposed between the flange 7 and the inlet tube 5. A tube 9 made of stainless steel is mounted around the front end of the inlet tube 5 that is located inside the ionization chamber 2. A porous member 10 is mounted to the tube 9 so as to plug the open end of the inlet tube 5. Since a repeller voltage is applied to the tube 9 via an electrode R, an insulating ring 11 that is mounted in the ionization chamber 2 is fitted over the tube 9. As an example, the porous member 10 is a filter made from a frit as produced by sintering powdered stainless steel. The effluent emerging from the chromatograph 6 is introduced through the inlet tube 5 and the porous member 10 into the ionization chamber 2. The beam generator 4 directs a beam B, such as a neutral particle beam, charged particle beam, or laser beam, onto the porous member 10 to ionize the introduced effluent. The resulting ions I are passed through the slits 3 into a mass analyzer (not shown).
The flow rate of the effluent from the chromatograph 6 ranges from 10 to 100 .mu.l/min., for example, while the flow rate of the effluent that can be admitted into the ionization chamber 2 is approximately 1 .mu.l/min. Therefore, the fraction, i.e., 9/10 to 99/100, of the effluent that cannot be entered into the ionization chamber is discharged to the outside via an exhaust pipe 12 and a flow control valve 13. In this system, either the variations in the flow rate occurring at the flow control valve 13 or the variations in the flow rate of the effluent from the chromatograph 6 due to pulsations produced by the pump that delivers the liquid greatly affects the flow rate of the effluent which is introduced into the ionization chamber via the porous member 10.
It is now assumed that the flow rate of the effluent from the chromatograph 6 is 101 .mu.l/min. and that the effluent passes into the porous member 10 and the exhaust pipe 12 at flow rates of 1 .mu.l/min. and 100 .mu.l/min., respectively. When the flow rate at the control valve 13 changes by only about .+-.0.5%, or 0.5 .mu.l/min., because of the variations in the operating conditions such as temperature, the change in the flow rate of the effluent introduced into the ionization chamber through the porous member 10 reaches as high as 50%, i.e., 1.+-.0.5 .mu.l/min. A similar undesirable situation takes place when the flow rate of the liquid delivered from the chromatograph 6 varies. Therefore, the amount of the produced ions also varies conspicuously, thereby impeding analysis. Further, many other problems, including large variations in the pressure inside the ion source, take place.